#define _CRT_SECURE_NO_WARNINGS 1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

class Solution {
public:
    TreeNode* convertBST(TreeNode* root)
    {
        int sum = 0;
        _convertBST(root, sum);
        return root;
    }

private:
    void _convertBST(TreeNode* root, int& sum)
    {
        if (root == nullptr) {
            return;
        }

        _convertBST(root->right, sum);

        sum += root->val;
        root->val = sum;

        _convertBST(root->left, sum);
    }
};